One of the essential steps in navigating a bone with MEMS sensors is to locate the bone relative to the sensors. For the femur, the orientation of the sensor relative to the lateral axis can be constrained mechanically, for instance, with claws inserted under the posterior condyles so that the sensor lateral axis is aligned with the lateral axis of the bone. However, the orientation of the sensor relative to the femoral mechanical axis is more complex, as one of the anatomical landmarks defining the axis, the femoral head, is hidden inside the hip.
In an optical navigation system, the femoral head is located by moving the femur and, assuming the pelvis is stable, finding the fixed pivot point around which the bone rotates. This relies on the optical sensor having six degrees of freedom (DOF), i.e., 3DOF in position and 3DOF in orientation.
However, in a MEMS system, sensors do not automatically provide 6DOF. The 6DOF can be retrieved by integrating gyroscope and accelerometer readings—a standard technique called “dead reckoning”—but this technique is very sensitive to sensor errors and thus ill suited to low-cost sensors. Other gyroscope-based methods for retrieving the axis of the femur based on an axial rotation kinematic exist. However, such methods require a very specific and not natural leg motion, which might be difficult to apply and constrain.